1. Field of the Invention
The present invention relates to a method and an arrangement for operating a rotating starting shear operating in a start-stop operation on rolling stock travelling at different speeds in wire mills, light-section mills, medium plate mills and billet mills, wherein the shear, whose gear system is connected through a coupling to a drive motor, is accelerated from the position of rest to approximately rolling speed and, after the cut has been carried out, is stopped again.
2. Description of the Related Art
It is important in the operation of shears of this type that the shear knives have at least during the cut such a circumferential speed that it coincides with the exit speed of the rolling stock to be cut, because otherwise the cut could not be carried out properly and problems in the operation would occur.
Conventional shears, for example, the drum shears described in German patent applications 30 20 084 A1 and 30 42 171 A1, have two knife drums which are commonly driven by a motor through a gear arrangement. In such shears, which must cover a large range of speeds, this has the following disadvantages:
a) High rates of rotation of the motor occur in the case of high rolling stock speeds with thin rolling stock. The available high kinetic energy in the cutting area is not required, but inevitably leads to the requirement of a high drive power in spite of the low cutting work required. PA1 b) In the case of low speeds of the rolling stock and thick rolling stock, the rates of rotation of the motor for the shear are also low. The low kinetic energy available in the cutting area frequently is not sufficient for carrying out the cutting work. Therefore, for eliminating this difficulty, it is necessary to select a larger motor with an appropriately higher drive power, which means that the motor is overdimensioned in the case of thin rolling stock and high rolling stock speeds. PA1 a) Starting the shear with a low drive torque which corresponds approximately to 10% of the rated torque, during the short period of time which is required for eliminating the play of the gear unit and the coupling. The duration of the start-up depends on the size of the play of the gear unit and the coupling and is about 10 ms. This measure according to the present invention prevents a high impact load on the shear shortly after the start-up; the impact-free start-up of the shear substantially suppresses additional vibrations and overloads of the shear mechanism and the service life of the parts transmitting forces is significantly increased. PA1 b) Constant acceleration of the shear after the start-up with maximum inclination and maximum acceleration torque to the desired speed of the shear for all rolling stock speeds and rolling stock cross-sections, wherein the desired shear speed is selected greater than the speed of the rolling stock (leading shear). Consequently, the full acceleration torque is available already at the beginning of the cut even in the case of large rolling stock cross-sections which require increased torques for cutting. PA1 c) Beginning the cut shortly before reaching the desired speed of the shear at a point in time at which the shear speed is already greater than the speed of the rolling stock, in order to ensure that the full acceleration torque is available already at the beginning of the cut. This measure according to the present invention makes it possible that when the rate of rotation drops no time is lost for again building up this torque and the drop of the rate of rotation itself is also minimized. PA1 d) Automatic monitoring and controlling of the circumferential speed of the shear knives during the cut in order to maintain an optimum lead of the shear. The automatic monitoring of the circumferential speed of the shear knives during the cut makes it possible in a simple and quick manner for the operator of the shear to evaluate in the case of problems, for example, when bending of the rod tips occurs, whether the speed of the shear knives is the reason of this problem as a result of a drop in the rate of rotation and, in this manner, the operator is able to determine more quickly the actual reason for the problem.
Another disadvantage of drives constructed in this manner is the fact that the cut is carried out only after the respectively required maximum shear speed has been reached, which means that the maximum motor torque is not available at the beginning of the cut. Consequently, the rate of rotation of the motor initially drops and the desired rate of rotation and the maximum motor torque are only reached again toward the end of the cut.